Scaffolding generally comprises a plurality of vertical support members or "standards", a plurality of horizontal bracing members, and a plurality of diagonally positioned cross members. Each of these members is generally formed from an elongate tubular section of pipe typically adapted at each end for interconnecting the ends to other scaffolding members. Elongate scaffolding members can thus be easily stored and transported to the desired construction site, where assembly is performed. After use, such scaffolding may be easily disassembled for storage or transportation to another site.
Many types of devices exist for interconnecting horizontal bracing members and diagonal cross members to support members, such as the devices disclosed in U.S. Pat. Nos. 4,044,523; 4,140,414; and 4,331,218. Although safety is paramount in the design of scaffolding, connection devices which have achieved widespread acceptance in the industry have appropriately balanced safety with ease of assembly and disassembly, manufacturing costs, and repair costs.
Interconnection of one vertical standard to another in order to achieve the desired platform height for the scaffolding is, in many respects, functionally dissimilar to other scaffolding interconnections. Scaffolding standards are the support members, and therefore must uphold the weight of the personnel and equipment on the scaffolding platform. When assembled, this interconnection is designed to functionally achieve a vertical unitary elongate support member, and reliability with respect to proper axial alignment of two assembled vertical standards is critical.
One commonly used technique for interconnecting vertical scaffolding support members involves the use of a male component having a pair of dogs connected to the end of one standard and a female component secured to an end of another standard and adapted to receive the male component. The male component is inserted in the female component and twisted together (generally requiring 1/4 turn or less) to become locked. This twistlock interconnection has advantages of easy assembly and disassembly. Also, the interconnction is made without portions protruding from the diameter of the standard, thus minimizing damage and corrosion to the interconnection components. Such a twistlock interconnection for vertical standards is shown in a brochure entitled "Safway Tube and Clamp Scaffolding, Catalog S-30, 1979", hereby incorporated by reference.
The above described twistlock connection for scaffolding has, however, several significant disadvantages. First, the interconnection components are typically of cast-metal construction, and the components are generally inserted in the ends of a tubular member and the tubular member crimped to retain the connection components. This crimping operation, although relatively inexpensive when performed at the scaffolding manufacturing plant, cannot be reliably performed in the field without an expensive press. Accordingly, field repair of such scaffolding standards is either very expensive or impossible. As a practical matter, repair of such scaffolding standards in the field is rare, with the result being the frequent usage of defective scaffolding standards.
Secondly, the crimping operation described above cannot be reliably controlled with respect to structural integrity. In other words, the cold metal deformation required according to this crimping operation does not provide repeatable and calculatable maximum axial and radial load exposure values between the tubular member and the respective interconnection component. Accordingly, a single connection component/tubular member connection may be vastly oversized from a structural integrity standpoint, but this oversizing or high safety factor is required to obtain reasonable reliability throughout a number of connections.
Perhaps the most serious drawback to the crimped connection of tubular members to such components, however, relates to safety. The tubular member is, of course, weakened by the crimping operation. This connection, although usually initially rigid, tends to become loose, so that the cast component may move slightly axially and radially with respect to the pipe. This loosening cannot, however, be easily tested or visually detected. Over a period of time and use, the cast component/tubular member connection may become so loose as to cause structural failure of the scaffolding. Moreover, when this connection becomes loose, assembled sections of pipe may not be axially aligned or plumb, thus substantially reducing the load bearing ability of the assembled scaffolding. Finally, this non-rigid connection may not uniformly distribute the axial load between the cast component and the end of the tubular member. Scaffolding tubular members are conventionally of the seam-welded type, and thus a concentrated load may be applied to the end of the pipe at the point of the weld, causing the tubular member to split. The frequency and severity of a total or partial scaffolding failure may, unfortunately, be appreciated from a brief review of related accident reports.
The disadvantages of the prior art are overcome by the present invention, and improved methods and apparatus is hereinafter described for forming a reliable twistlock interconnection for vertical scaffolding support members.